Fluidized beds are often used in municipal solid waste incinerators. The typical fluidized bed incinerator includes several hundred air distributor nozzles located at the bottom of the bed and welded to an air distribution plate. This plate normally has a layer of high temperature refractory material on it and silica sand serving as a fluidizing medium. The typical nozzle is mushroom shaped with numerous drilled holes in the top surface thereof.
The fluidized sand, every time the incinerator is shut down, tends to lodge inside the drilled holes. Then, at the restart of the incinerator, there is a pressure rise in the plenum of the bed as well as uneven fluidization and, with time, even distortion of the air distributor plate. This condition may force a shut down of the solid waste incinerator. The inventor hereof discovered that this phenomenon occurs because, after every shutdown, the air in the plenum of the bed cools faster than the sand above the air distribution plate that holds the nozzles. The hydraulic pressure of the sand in the bed is greater than the pressure in the plenum, and this phenomenon sucks the sand into the holes of the mushroom shaped nozzles plugging them.
Unplugging all of the holes, one by one with carbide drills, involves a lengthy and labor intensive and thus expensive shutdown of the solid waste incinerator. The typical shutdown process includes a cool down time, the removal of all the sand from the bed, the manpower required to drill each hole in all the nozzles (a process often taking days as there are approximately 19,000⅛ inch holes in a 12 foot diameter incinerator), reloading the sand, and start up of the incinerator. Depending upon the frequency of scheduled incinerator shutdowns, plugging of the nozzle holes can force an unscheduled maintenance every three or four months.